The fuel rods for a nuclear reactor generally comprise cladding of tubular shape and, inside the cladding, pellets of fuel material, generally made by sintering, which are stacked along the longitudinal direction of the tubular cladding.
Owing to the swelling of the fuel material pellets under irradiation, the peripheral part of the pellets comes into contact abutment, with a certain pressure, against the internal surface of the cladding, when the fuel is used in the core of a nuclear reactor.
This may result in interactions between the pellets and the cladding which may lead to damage of the cladding, promoted by the working temperature of the fuel.
It has been proposed to limit the possibilities of contact between the pellets and the cladding, in order to avoid as far as possible the drawbacks linked with this phenomenon which is called pellet-cladding interaction (PCI).
It has been proposed, for example, in the case of fuel for a reactor cooled by boiling water, to use cladding tubes whose internal surface comprises longitudinal grooves, so as to limit the area of contact between the pellets and the cladding, while allowing sufficient thermal transfer of the heat produced by the nuclear fuel to the outside of the cladding.
In the case of rods for fuel assemblies intended for pressurized water nuclear reactors, it has also been proposed to use tubular cladding whose prismatically-shaped internal surface consists of faces, all having the same width, which are arranged successively and regularly over the internal surface of the cladding.
With such a configuration, the fuel pellets which have a circular contour, are capable of coming into contact with the faces of the internal surface of the cladding, in zones which are isolated from one another, along the generatrices of the pellets.
According to this principle, cladding is produced which comprises thirty or forty faces over its internal surface.
In the context of development studies and manufacture of tubular cladding comprising faces over its internal surface, it is necessary to provide a method for examining the faces on the internal surface of the tubular cladding.
In particular, it is necessary to examine the number of faces over the entire internal periphery of the tube and the width of the faces in the peripheral direction. An examination must indeed be made as to whether all the tubes leaving manufacture have the same number of internal faces and whether the faces have the same amplitude over the periphery of the internal surface of the cladding and over the entire length of the tube.
In order to examine the thickness of the fuel rod cladding at the end of manufacture, it is known to use an ultrasonic examination head of annular shape which is engaged in a coaxial position around the tubular cladding to be examined and which is rotated about its axis at high speed.
The ultrasonic examination head comprises one or more transducers which emit ultrasound waves in substantially radial directions towards the inside of the tube. In order to carry out the thickness examination of the tube continuously or in successive zones along its length, a relative displacement of the tube and of the rotating examination head is produced so as to scan the entire peripheral surface of the tube along a helicoid trajectory by a beam of ultrasound waves.
Analysis of the signal corresponding to the ultrasound waves reflected from the internal surface of the tubular cladding makes it possible to carry out a thickness measurement and examination of the tubular cladding.
When use is made of ultrasound waves which are emitted in the form of successive pulses or wave trains, measurements can be made at isolated points distributed over the periphery of the tube, the number of measurement points on the periphery of the tube, i.e., over one complete revolution, depending on the speed of rotation of the examination head and on the repetition frequency of the ultrasound wave trains. However, such a measurement procedure has never been used for examining the number and the amplitude of the internal faces of a multi-faced cladding tube. Furthermore, if such a method were to be used for examining faces, it would prove unusable insofar as the speed of rotation of the examination head and the constancy of this speed over time cannot be guaranteed with sufficient precision. For this reason, it is not possible to make accurately localized measurements with respect to the faces of the internal surface of the tubular cladding.